1. Technical Field
The disclosure relates to wireless communication and, more particularly, techniques for signal-to-noise ratio estimation.
2. Related Art
A widely used technique for wireless communication is code division multiple access (CDMA) signal modulation. In a CDMA system, multiple communications are simultaneously transmitted between base stations and mobile subscriber units over a spread spectrum radio-frequency (RF) signal. In CDMA and other spread spectrum systems, maximizing system capacity and maintaining quality of service are paramount concerns. System capacity in a spread spectrum system can be maximized by carefully controlling the transmit power of each subscriber unit and base station in the system.
If a signal transmitted by a subscriber unit arrives at the base station at a power level that is too low, the bit-error-rate may be too high to permit effective communication with that subscriber unit, undermining quality of service. On the other hand, signals with power levels that are too high can interfere with communication between the base station and other subscriber units in the system, reducing system capacity. For this reason, to maintain system capacity and quality of service, it is desirable to monitor and control the level of transmit power of signals transmitted by base stations and subscriber units.
The subscriber units and base stations typically communicate with one another to control the transmit power of signals sent in the system. For example, to control the level of transmit power in a base station, the subscriber unit estimates the power of a signal received from the base station and transmits instructions, such as power control bits, to the base station via a control channel. The base station adjusts transmit power based on the power control bits transmitted from the subscriber unit.
The process for controlling base station transmit power is often referred to as forward power control (FPC). For effective forward power control, the subscriber unit estimates the signal-to-noise ratio of signals received from the base station. The signal-to-noise ratio is typically expressed as the ratio Eb/Nt, i.e., the ratio of signal energy per information-bit (Eb) to noise power spectral density (Nt).